The present invention relates in general to circuits for deploying airbag igniters or squibs, and, more specifically, to circuit diagnostics for testing proper operation of drivers that supply electrical energy to ignite the squibs.
The main components of an airbag supplemental restraint system used in motor vehicles include an inflatable bag, a propellant source (e.g., sodium azide pellets), an igniter or squib to initiate burning of the propellant source, at least one crash sensor, and an electronic control module for determining when to deploy an airbag and sending a deployment pulse to the igniter. The airbag, propellant, and igniter are contained in an airbag module (e.g., within a steering wheel for a driver airbag). The sensor can be packaged separately or can be contained within the electronic control module.
The control module performs self-diagnostic monitoring of the supplemental restraint system each time the system is turned on (e.g., every time a vehicle is started). Any potential performance problems are identified and a warning light is illuminated so that the driver knows that the system needs to be serviced.
It is known to perform diagnostic monitoring of the electrical connection of the squib elements, squib resistances, and electrical leakage in the squib circuits, among other tests. When performing electrical testing involving the squibs, care must be taken to avoid application of any current to a squib that could cause inadvertent deployment of the airbag. Due to the cost of replacing an airbag module and the loss of supplemental protection until replacement occurs, diagnostic monitoring should not increase the chances of inadvertent deployment.
A very desirable test to be able to perform is a driver test in which a squib driver circuit can be activated in a test mode without igniting the squib. Such a test can verify that a semiconductor switch in series with the squib element itself will conduct as intended during an actual deployment event. However, such a test has been problematic since the activation of the switch partially completes the deployment circuit. If certain other faults exist, or if switch activation is not implemented properly, unintended deployments can occur.
The present invention has the advantage that high-side and low-side drivers in series with a squib element can be tested while avoiding inadvertent airbag deployment.
In one aspect, the present invention provides a method of testing a high-side driver and a low-side driver in an airbag squib circuit. The airbag squib circuit includes a squib element coupled between the high-side driver and the low-side driver. The high-side driver controllably provides a high-side voltage to one side of the squib element and the low-side driver controllably provides a low-side voltage to the other side of the squib element. A resistance of the squib element is tested for a resistance value within a predetermined resistance range. A current leakage associated with said squib element is tested to determine whether it is over a leakage threshold. An intermediate voltage from a weak power supply is supplied to a point in the airbag squib circuit between the high-side driver and the low-side driver. One of the drivers is turned on while keeping the other one of the drivers off. A voltage at the point is continuously compared with a predetermined voltage range which includes the intermediate voltage. The one driver is turned off in response to the voltage at the point being outside the predetermined voltage range, thereby detecting that the one driver is operating properly. If the voltage at the point remains in the predetermined voltage range for a predetermined time period, then the one driver is turned off and an indication is made that the one driver has failed.
Unless there is a failure, the other driver is then turned on while keeping the one driver off. A voltage at the point is continuously compared with the predetermined voltage range. The other driver is turned off in response to the voltage at the point being outside the predetermined voltage range, thereby detecting that the other driver is operating properly. If the voltage at the point remains in the predetermined voltage range for the predetermined time period, then the other driver is turned off and an indication is made that the other driver has failed.